1. Field of the Invention
The present invention relates to a printer, and more particularly, to a device for preventing a header of an ink-jet printer from overheating.
2. Description of the Related Art
An ink-jet printer is designed to produce a desired image on paper by ejecting ink onto the paper. The ink-jet printer includes a header providing ink to a plurality of nozzles through which the ink is ejected, and an electric circuit device that is designed to selectively operate the nozzles of the header according to printing data.
The ink-jet printer is classified into one of a piezo type printer and a bubble jet type printer by an ink discharging method. While the piezo type printer discharges the ink onto the paper by pressing an ink path, which the ink flows into, using a pressure element, the bubble jet type printer discharges the ink onto the paper by changing a volume of an ink drop which is formed by super-heating an ink discharge portion.
FIG. 1 is a view schematically showing a circuit of a general bubble jet type ink-jet printer. The ink-jet printer includes a printer system card 10 electrically controlling general operations of a system and a header 20 having a heater RH that emits heat to form an ink drop in response to a control signal and a driving voltage Vph transmitted from the printer system card 10.
The printer system card 10 includes a main process unit (MPU) 12 controlling the general operations of the system and a first transistor FET1 switching the driving voltage Vph to drive the heater RH of the header 20 under a control of the MPU 12. The header 20 has a second transistor FET2 that is driven by the control of the MPU 12, and the heater RH that emits the heat when the FET2 is driven. Generally, the heater RH consists of a resistance and is built in a substrate or a nozzle plate. Although FIG. 1 shows a single heater RH and a single FET2 corresponding to one ink discharging opening by way of example, all of ink discharging openings are individually provided with the heater RH and the transistor FET2.
In the bubble type ink-jet printer as constructed above, the MPU 12 drives the FET1 according to transmitted printing data to supply the driving voltage Vph to the heater RH, and the MPU 12 also drives the FET2 such that the heater RH emits the heat. Accordingly, an ink drop is generated by the heater RH emitting the heat, and a volume of the ink drop becomes gradually larger. When the ink drop reaches a limit such that the ink drop does not become larger, the ink drop is pushed toward an ink discharging opening and discharged onto the paper. At this point, the ink is optimally discharged when a temperature of the ink is approximately 40° C. Therefore, the MPU 12 controls the FET2 supplying current electricity to the heater RH for a predetermined time to allow the substrate and the nozzle plate having the heater RH to reach the temperature of 40° C.
The general bubble type ink-jet printer heats the nozzle plate or the substrate at an optimum temperature under a normal condition, but it has a problem of overheating the heater under an abnormal condition, i.e., when there occurs an abnormality of the MPU in detecting the temperature. As the result, the nozzle plate or the substrate melts or overheats.
In order to solve the above problem, the MPU of the prior art detects a temperature of the header 20 through a temperature detecting unit and stops operating the FET1 shown in FIG. 1 to protect the header 20 from overheating when the detected temperature reaches a predetermined temperature.
However, the above conventional method of preventing the header from overheating using a software-like process still has a problem in that the substrate or the nozzle plate is overheated when an abnormality occurs in detecting the temperature.